Sometimes it's not possible to know what bit is needed, like with TA30s or TA27s until you try various sizes in the hole.
I could get one that's about 9.75mm on amazon, then chuck it in the drill press and reduce it with a file if it's still too large. I'm thinking of small appliances that often have various types of "security" fasteners. You can't tell what bit you need until you find it necessary to disassemble some of this stuff.